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Cretaceous shallow-water limestones were recovered from two flat-topped submarine seamounts (guyots) in the area of the Mid-Pacific Mountains during Leg 143. In the center of Allison Guyot (Site 865), a nearly 700-m-thick sequence of upper Albian lagoonal limestones overlain by a 150-m-thick pelagic sedimentary cover of Paleogene to Quaternary age was drilled before terminating in basalt. A transect from the lagoon across the perimeter mound of Resolution Guyot was established by Sites 866, 867, and 868. Hole 866A resulted in a thick (more than 1600 m of carbonates overlying basalt) sequence of Hauterivian to Albian lagoonal to reefal limestones. Resolution Guyot bears only a thin pelagic cap of Maastrichtian to Pliocene age. The diagenesis of the shallow-water Lower Cretaceous carbonates of Allison and Resolution guyots took place in response to changes in rock pore-fluid composition over time. Analyses of diagenetic variability in combination with geochemical studies allow for the reconstruction of chemical compositions and diagenetic pathways. The disappearance of the volcanic island is evidenced by the decreasing clay contents of the limestones. Major, minor, and trace element analysis of bulk samples illustrates this process in high resolution. We identified an excellent correspondence of cyclic distributions of phosphate, manganese, copper, and zinc with sequence-stratigraphic interpretations for Site 866: primary element compositions are probably related to sea-level variations. Early diagenetic alterations are documented by different cement and porosity types. Analyses of cement chemistry and stable isotope composition permit the reconstruction of marine-phreatic, meteoric-phreatic, and meteoric-vadose diagenetic environments, prior to the development of karst at the top of Hole 866Aand the strong late diagenetic dolomitization in its lower part.

Diagenetic Alterations and Geochemical Trends in Early Cretaceous Shallow-Water Limestones of Allison and Resolution Guyots (Sites 865 to 868)